Method for making single-phase anatase titanium oxide

ABSTRACT

This invention relates to methods of making single phase nanocrystalline titanium dioxide. It is hereby provided a method for preparing single-phase anatase type titanium dioxide photocatalyst having a particle size of nano level at near room temperatures without the need for a sintering process at high temperatures.

FIELD OF THE INVENTION

The present invention relates to methods for preparing single-phaseanatase type titanium dioxide having a particle size of nano level atlow temperatures and, more particularly, to a method for preparing aphotocatalyst without the need for a high-temperature sintering process.

BACKGROUND OF THE INVENTION

Titanium dioxides are useful materials due to their interestingproperties and important applications. A common use for Titanium dioxide(TiO₂) is as a photocatalyst. In order to be used as a photocatalyst,titanium dioxide has to exhibit crystallinity. Among the crystallinephases of titanium dioxide, anatase is reported to show the highestperformance. The preparation of anatase thin films is usually achievedat temperatures above 400.degrees celsius. However, the need for hightemperature processing limits the choice of substrates to heat resistantmaterials. The growth of anatase titania nanocrystallites at lowtemperatures is important for the fabrication of nanocrystalline titaniafilms on substrates with low heat resistance, such as wood, paper,plastics, textiles and biomaterials.

Thus, in a case where amorphous titanium dioxide is formed from atitanium-based starting material through hydrolysis and condensationpolymerization, it is a necessity to carry out a heat treatment at hightemperature, i.e., sintering process, in order to convert the amorphoustitanium dioxide to an anatase type titanium dioxide. It is known thatthe sintering temperature is typically around or above 400.degreescelsius.

Accordingly, it is fundamentally impossible to form anatase titania or acomposite oxide containing anatase titania, for example, on substrateshaving low resistance to heat such as organic polymer substrates orliving tissue having low resistance to high temperature, low pressureand high pressure.

It has been reported that anatase nanocrystallites can be grown oncotton fabrics using a boiling water treatment (W. A. Daoud and J. H.Xin, J. Am. Ceram Soc., 87 [5] 953-55 2004). Even though, the processingtemperature could be reduced to 100 degrees celsius, the need for apost-preparation heat treatment is still there. However, the in-situpreparation of anatase oxides at temperatures of low temperatures is ofgreat importance not only from the point of view of energy saving, asannealing post-treatment is avoided, but also it would open up newapplications in new fields such as biochemistry.

Among conventional methods using titanium dioxide as a photocatalyst,some can be distinguished as the most widely used methods: a firstmethod of using an anatase type titanium dioxide in the powder form anda second one of using a thin film of the anatase type titanium dioxideformed on a specific support. Although the former is superior to thelatter in the photocatalyst activity due to the relatively large surfacearea of the titanium dioxide prepared, the latter is more practicable inthe aspect of stability of the titanium dioxide. Therefore, the actuallyused photocatalysts are prepared by the second method that involvesformation of a titanium dioxide film on a support by means of a sol-gelmethod.

The process used in conventional methods to prepare anatase materialsincludes the steps of: (a) preparing a titanium oxide precipitate froman aqueous solution of a titanium-based starting material by hydrolysisand condensation polymerization; (b) subjecting the precipitate tofiltration to obtain a white amorphous titanium dioxide; (c) sinteringthe amorphous titanium dioxide at high temperature (above 400 degreescelsius.) to obtain an anatase type titanium dioxide; (d) milling theresulting titanium dioxide to yield a powdery titanium dioxide; (e)dispersing the powder in a specific solvent to prepare an anatase typetitanium dioxide solution; and (f) coating the solution on a support.

Such a conventional method for preparing a titanium dioxidephotocatalyst is a multistage process that involves the steps offiltration, sintering, milling and dispersion, which may cause anincrease in the unit production cost. The conventional method is also atroublesome in that there is a need of milling the anatase type titaniumdioxide into very tiny particles, i.e., in the particle size of nanolevel, and dispersing the particles in a specific solvent prior to acoating step, in order to enhance the outer appearance of the finallycoated photocatalyst or to increase the surface area of thephotocatalyst. With a large particle size of the anatase type titaniumdioxide, an excessively large amount of the precipitate may be formed inthe dispersion step and the resulting photocatalyst is inapplicable to acoating solution.

Also, the use of several additives for enhancing the coatingcharacteristics and the hardness of the photocatalyst films may cause aproblem such as deteriorating the stability of the titanium dioxidedisperse solution and resulting in formation of precipitates.

The most well-known anatase type titanium dioxide powder commerciallyavailable is P25 supplied by Degussa, which is now manufactured in alimited number of countries. There are also commercially availablecoating products containing a photocatalyst dispersed in water andethanol, which are too expensive due to their expensive preparationprocess.

OBJECTS OF THE INVENTION

Therefore, the invention of this application solves the aforesaidproblems of the related art, and aims at providing a novel process whichcan produce anatase titania at a low temperature and, in addition, atatmospheric pressure, thereby can be used to form coating films on avariety of substrates including polymer materials or living tissuehaving low resistance to heat, and moreover can form anatase titania ina variety of forms such as bulk, film, coating, fiber, powder, etc.

DISCLOSURE OF THE INVENTION

Accordingly, this invention provides a method of preparing single-phaseanatase type titanium dioxide photocatalyst having a particle size ofnano level at temperatures below 58 degrees celsius and, moreparticularly, to a method for preparing a photocatalyst without the needfor a sintering process or other post-deposition treatments.

To achieve the objectives of the present invention, the hereby disclosedmethod includes adding a titanium-based starting material to a selectedsolvent containing an acid or base catalyst and an organic acid. This isfollowed by subjecting the catalyst-containing solution to a heattreatment at at temperature of between 35 and 58 degrees celsius toactivate the crystallization thereby preparing an anatase type titaniumdioxide sol solution. Finally, the anatase type titanium dioxide solsolution is coated onto a support to complete the preparation of thephotocatalyst films or made to precipitate into anatase powder.

In particular, according to the invention of this application, atransparent film of anatase titania which has been difficult to from inthe related art can be formed on a variety of substrates using a lowtemperature process, and therefore the fields of application areextremely wide and promising.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

This invention is now described by way of example with reference to thefigures in the following paragraphs.

Objects, features, and aspects of the present invention are disclosed inor are obvious from the following description. It is to be understood byone of ordinary skill in the art that the present discussion is adescription of exemplary embodiments only, and is not intended aslimiting the broader aspects of the present invention, which broaderaspects are embodied in the exemplary constructions.

The method of coating titanium oxides to an article according to thisinvention may be divided into two steps. The first step involves theforming of a colloidal solution of single-phase titanium dioxide oxideby hydrolysis and condensation of a titanium precursor in a solvent. Thetitanium precursor used in the making of the sol mixture can be varioustitanium compounds, for example, titanium alkoxides, such as titaniumtetraisoproxide, titanium tetrabutoxide; titanium nitrate; titanylsulfate, titanium halides, such as titanium chloride.

The solvent used in this invention is to provide a suspending medium.Water is the main medium used. In this case, a direct hydrolysis of thealkoxides occurs to produce hydroxide compounds. This is followed by apolycondensation reaction to give titanium oxide product. A small amountof an alcohol can also be added in order to accelerate the hydrolysisstep. Ethanol may be a preferred choice as it is environmental friendly,available and low cost, but of course other alcohols can be used.Ethanol can react with the alkoxides such as titaniumtetraisopropoxideto give titanium tetraethoxide. This may accelerate the hydrolysis stepof the alkoxide group to give a hydroxide product.

Acids and bases may be used to as catalysts for the hydrolysis andcondensation of the titanium precursors. Organic acids, such as aceticacid or formic acid are used as crystallization-accelerating andstabilizing agents.

The sol mixture may be prepared, at temperatures of between 35 and 58degrees celsius by mixing the titanium dioxide precursor with thesolvent. The mixture is then stirred for a period of time prior tocoating. The time of preparation is optional and entirely depending onthe phase and size of crystals. Short reaction time may lead to theformation of anatase and brookite mixture. The use of the high end ofthe preparation temperatures may reduce the reaction time in order toform single-phase anatase, however it may produce larger nanocrystals.The following equations may summarize the principal reactions involved:

Ti(OPr)₄+4 EtOH→Ti(OEt)₄+4PrOH

Ti(OPr)₄ or Ti(OEt)₄+H₂O→Ti(OH)₄+4PrOH or 4EtOH

Ti(OH)₄.→TiO₂+2 H₂O

The article is then coated with the anatase colloidal sol solution usingvarious deposition techniques such as dip-coating, spin-coating, solventcasting, spraying, doctor blading, etc.

The article may then be dried in air and finally cured at temperaturessimilar to the preparation temperature range for a certain period oftime in order to develop the inorganic network and densify the coatingsand thus improve the interfacial surface adhesion to the article. Thesecuring time and temperature after coating are entirely optional.

Example 1

In a 500 ml beaker, 240 ml water, 60 ml ethanol, 60 ml acetic acid, and3 ml nitric acid were mixed and stirred rapidly. To this, 7.5 mltitanium tetraisopropoxide were added while stirring. The formedtransparent mixture was then heated to 38 degrees celsius undercontinuous stirring for 12 hours. The resulting solution was colourlessand transparent. A silicon wafer substrate and a non-alkali glasssubstrate were coated with the aforesaid solution by spin coating at1500 rpm for 50 seconds and the respective substrates were air dried forone hour and then oven dried at 38 degrees celsius for 4 hours.Observations of high resolution transmission electron microscope showedthat the formed films were of pure anatase phase according to thelattice fringe analysis which shows a lattice spacing of 0.352 nm thatis associated with the anatase plane (101). When 1 ml of sodiumcarbonate solution (10% weight) was added to 10 ml of the preparedanatase solution, white solid precipitated. This was collected bycentrifugation and filtration. The solid powder was then washed oncewith water and once with acetone and finally dried at 38.degrees celsiusfor 4 hours. X-ray diffraction analysis of the resulting powderconfirmed that the product is pure anatase.

Example 2

In a 500 ml beaker, 240 ml water, 60 ml ethanol, 60 ml acetic acid, and3 ml nitric acid were mixed and stirred rapidly. To this, 7.5 mltitanium tetraisopropoxide were added while stirring. The formedtransparent mixture was then heated to 58 degrees celsius undercontinuous stirring for 8 hours. The resulting solution was colorlessand transparent. A silicon wafer substrate and a non-alkali glasssubstrate were coated with the aforesaid solution by spin coating at1500 rpm for 50 seconds and the respective substrates were air dried forone hour and then oven dried at 38 degrees celsius for 4 hours.Observations of high resolution transmission electron microscope showedthat the formed films were of pure anatase phase according to thelattice fringe analysis which shows a lattice spacing of 0.352 nm thatis associated with the anatase plane (101). When 1 ml of sodiumcarbonate solution (10% weight) was added to 10 ml of the preparedanatase solution, white solid precipitated. This was collected bycentrifugation and filtration. The solid powder was then washed oncewith water and once with acetone and finally dried at 38.degrees celsiusfor 4 hours. X-ray diffraction analysis of the resulting powderconfirmed that the product is pure anatase.

The Novel Preparation Method has the Following Advantages

(1) The method eliminates a need of the sintering and milling steps.That is, the single-phase anatase type titanium dioxide having a size ofnano level can be prepared by a single integrated step of filtration,sintering, milling and dispersion according to the present invention.Thus there is no necessity to perform a sintering step forcrystallization or a milling step of the resulting titanium dioxideagglomerate after the sintering.

(2) The method also eliminates the need of a dispersion step forpreparing a coating solution. In this method, the anatase type titaniumdioxide aqueous solution with a particle size of nano level ismaintained in a stable state for a long time and thus can be usedimmediately as a coating solution without any treatment. This makes itpossible to skip a step of dispersing the milled titanium dioxideparticles in the solvent to prepare a separate coating solution.

(3) In preparation of a titanium dioxide photocatalyst having differentproperties due to various additives, the invention allows a simpleprocess for the preparation while reducing the possibility of variousproblems arisen from the additives dispersed in the solvent, such aschemical reaction, precipitation and phase separation.

(4) The photocatalyst obtained in this preparation method is availableas a coating applicable to various types of supports in a simple way.According to the preparation method of the present invention, theanatase type titanium dioxide solution having a particle size of nanolevel exhibits an optical activity and thus can be coated on thesupports without any separate treatment. This coating on the supportforms the final photocatalyst only after a drying.

(5) Due to the much reduced processing temperatures of the presentmethod, thereby film coating of single phase anatase can be performed ona variety of substrates including polymer materials or living tissuehaving low resistance to heat, and moreover can form anatase titania ina variety of forms such as bulk, film, fiber, powder, etc.

As described above, the preparation method of the present inventionmakes it possible to provide an anatase type titanium dioxide having acontrollable particle size of nano level in a single step, the anatasetype titanium dioxide being-easily coated on a support. Namely, thepresent invention method eliminates the need for filtration, sintering,milling, dispersion and sintering in the preparation of a titaniumdioxide photocatalyst to make the preparation process simple withreduced production cost, as well as making it easier to introducevarious additives in the photocatalyst, thereby providing aphotocatalyst film with high strength applicable to almost all type ofsupports.

While the preferred embodiment of the present invention has beendescribed in detail by the examples, it is apparent to those skilled inthe art that modifications and adaptations can be made in the presentinvention without departing from the spirit or scope of the invention.Furthermore, the embodiments of the present invention shall not beinterpreted to be restricted by the examples or figures only. It is tobe expressly understood, however, that such modifications andadaptations are within the scope of the present invention, as set forthin the following claims. For instance, features illustrated or describedas part of one embodiment can be used on another embodiment to yield astill further embodiment. Thus, it is intended that the presentinvention cover such modifications and variations as come within thescope of the claims and their equivalents.

1-16. (canceled)
 17. A method for preparing an article coated withsingle-phase anatase titanium dioxide having a nano level particle size,comprising: adding a titanium-based starting material to a solventcontaining at least water and an organic acid; subjecting the resultingsolution to heat treatment at a temperature of between 35 and 58° C. toactivate crystallization and thereby obtaining a single-phase anatasetitanium dioxide sol solution, coating the article with the single-phaseanatase titanium dioxide sol solution, and drying the coated article ata temperature between 35 and 58° C.
 18. A method of treating an articlewith single-phase anatase titanium dioxide having a nano level particlesize; comprising preparing a single-phase anatase titanium dioxide solsolution by adding a titanium-based starting material to a solventcontaining water and an acid or base catalyst and an organic acid;subjecting the resulting solution to heat treatment at a temperature ofbetween 35 and 58° C. to activate crystallization and thereby obtaininga single-phase anatase titanium dioxide sol solution; coating thesingle-phase anatase titanium dioxide sol solution onto an article orisolating the single-phase anatase solids from the solution byprecipitation; and drying the coated article at a temperature between 35and 58° C.
 19. The method of claim 18 wherein the resulting solution isheat treated at a temperature of between 35 and 50° C.
 20. The method ofclaim 18 wherein the resulting solution contains a hydrolysable titaniumcompound.
 21. The method of claim 18 wherein the hydrolysable titaniumcompound is selected from the group consisting of titanium alkoxides,titanium halides, titanium nitrates, and titanyl sulfates.
 22. Themethod of claim 18 wherein the solvent is a solution comprising water,alcohol or a mixture thereof.
 23. The method of claim 18 wherein thealcohol is selected from the group consisting of methanol, ethanol,n-propanol, 2-propanol and butanol.
 24. The method of claim 18 furtherincluding stirring the solvent during the adding of the hydrolysabletitanium compound.
 25. The method of claim 18 wherein the catalyst is anacid catalyst.
 26. The method of claim 18 wherein organic acids areselected from the group of formic acid, acetic acid and propionic acid.27. The method of claim 18 further including stirring the resultingsolution while subjecting the resulting solution to heat treatment. 28.The method of claim 18 wherein the article is selected from the groupconsisting of glass, aluminum, steel, ceramics, polymers, wood, papers,textiles and biomaterials.
 29. The method of claim 18 wherein a pHbuffer solution is added to precipitate the anatase solid.
 30. Themethod of claim 18 wherein the precipitated anatase solids are collectedby centrifugation or filtration.